The present invention relates to a format structure of a magnetic disk by which the error correction capability can be improved without sacrificing the storage capacity, and more particularly to a magnetic disk drive in which the error correction capability is improved by adopting such a format structure.
As far as a magnetic disk drive that adopts a data-surface servo system is concerned (this system is also called a sector servo system or an embedded servo system), as shown in FIG. 7, a plurality of data sectors 2 through 5 are formed between servo sectors 1a, 1b in the circumferential direction of a data track 7 of a magnetic disk with each gap area G being inserted therebetween. The magnetic disk drive is configured as a direct access storage device capable of writing and reading data on a data sector basis. Each of the data sectors 2 through 5 includes: user data; an error correction code (ECC) that is generated on the basis of the user data; and other supplementary data. The data sectors 2 through 5 adopt fixed block architecture (FBA) in which the total length is constant. Because a data sector whose block length is fixed is so formed that the total number of bytes is constant, one data sector occupies, in the circumferential direction of the data track, a write area whose length is proportional to the rotational speed of the magnetic disk.
Another magnetic disk drive adopts an ID-less format system in which address information is eliminated from each data sector. The magnetic disk drive is configured such that pulses having a specified frequency are counted from a servo sector 1 or from another reference position by the number of pulses corresponding to the rotational speed of a magnetic disk for identifying a position of each of data sectors 2 through 5. When the data sector 3 is updated in a state in which data is written to the data sectors 2 through 5, it is necessary to prevent data forming the data sector 2 or the data sector 4 from being overwritten. The rotational speed of the magnetic disk is controlled so that the rotational speed is always kept constant. However, it is not possible to completely suppress the occurrence of rotation jitter. The rotation jitter means the rotation inaccuracy of a magnetic disk. In general, the rotational speed of the magnetic disk contains an error of about +/−0.2% to 0.5% with the specified speed being its center. If the rotation jitter occurs, there is a possibility that adjacent data sectors and a servo sector will be overwritten. Therefore, gap areas G which are not used to write data are formed between the data sectors and between the data sector and the servo sector. Because the gap areas exceed 1% of a recording surface of the magnetic disk, it is useful if the gap areas can be effectively utilized.
Incidentally, in recent years, the recording density of magnetic disk drives increases. As a result, a ratio of the occurrence of read errors also increases. A read error whose number of error bytes is a specified value or less is corrected by hardware processing using ECC, that is, by on-the-fly ECC. Therefore, the delay time of the processing is short, and read operation of the magnetic disk drive is seldom delayed. However, if a read error whose number of error bytes exceeds the error correction capability of ECC occurs, error recovery procedures (ERP) are executed. Therefore, the read error is corrected as a result of read retry including various kinds of recovery operation. Because the error recovery operation performed by executing the ERP takes a long time, it is desirable that a read bit error be corrected by on-the-fly ECC if possible.
Paying attention to the fact that an influence of jitter increases with the increase in the distance between a data sector and a servo wedge, patent document 1 (Japanese Patent Laid-Open No. 2001-273722) discloses a technology in which the length of a gap area formed between a servo wedge and a data sector close to the servo wedge is shortened. Patent document 2 (Japanese Patent Laid-Open No. 2002-124037) discloses a technology in which a gap area and a preamble are eliminated to increase the storage capacity by rewriting data not on a data sector basis, but on a data field basis or on a data track basis. The data field and the data track are constituted of a plurality of data sectors. In patent document 3 (Japanese Patent Laid-Open No. 2001-143406), FIG. 3 of the reference illustrates a format structure in which a data area of a sector is partitioned. In this format structure, two partitioned data areas are provided with only one preamble. A unit of writing is the whole area including the two data areas ranging from the preamble to ECC.
Patent document 4 (Japanese Patent Laid-Open No. 2004-253042) discloses a technology in which a servo-clock synchronization recording method is adopted, and thereby the gap length is shortened to minimize a useless area in a track format. A servo clock is generated by synchronizing dedicated PLL with a read signal of the clock marks. Here, clock marks included in servo areas are used as the reference time. The clock marks are radially formed at intervals of substantially the same angle on a disk. Various kinds of timing signals including a RG pulse and a WG pulse, which are synchronized with this servo clock, are generated. In addition, a bit clock used for writing data is also generated.